Understanding how RIF1 and KAP1 enable the choice of the future active and inactive X chromosomes: the establishment of functional asymmetry.

在人类中，每条染色体都有两个副本，但男性除外，每个性染色体都有一个副本，即X和Y。女性有两个X染色体副本。女性的两条X染色体中有一条被灭活(沉默)，因此男性和女性细胞具有相同数量的来自X染色体的基因产物。这种沉默的过程被称为X失活。这两条X染色体中的任何一条都可以随机灭活。这意味着，在每个组织中，一半的细胞会有一个X染色体不活跃(A)，另一半的细胞会有另一个不活跃(B)。最初是如何随机选择两条X染色体中的一条的？通知一条染色体另一条染色体被选择的机制是什么？在一个人的一生中，如何在每个细胞的后代中稳定地保持不同的选择？这些都是仍未得到回答或只有部分答案的问题。此外，在现实中，X染色体的两个副本永远不会完全相同，它们DNA序列中的微小差异可能会导致两个副本中的一个更容易被沉默。因此，例如，如果使X染色体A更容易沉默，那么人类器官中的大多数细胞都将具有活跃的B X染色体，而不是具有活跃A的细胞和具有活跃B的细胞的混合。当两条X染色体中的一条含有可能导致疾病的完全或部分缺陷基因时，这一点非常重要。如果优先沉默的X染色体携带疾病版本的基因，器官就会发挥功能，女性就会健康。但是，如果优先沉默的X染色体携带正常版本的基因，器官将无法正常运作，女性将患上这种疾病。因此，确定导致这种扭曲的DNA序列的特征并了解X-失活是如何工作的非常重要，因为它们将有助于在X-连锁疾病的女性携带者中及早识别出有可能出现严重症状的患者。这种疾病的一个例子是Duchenne肌营养不良症，其中偏斜的X-失活在症状的严重性中起到了作用，一些女性患者完全没有症状，另一些人病情严重。在我们的项目中，我们使用小鼠胚胎干细胞来研究三个分子，一个RNA和两个蛋白质，我们已经证明这三个分子对于初始选择哪个X染色体将被失活是必不可少的。通过研究这三个分子如何相互作用以及与X染色体DNA的相互作用，我们将确定建立未来活动和非活动X染色体身份选择的分子机制。此外，我们将探索DNA序列的作用，这些序列可以确定扭曲的X染色体沉默，并调查这些序列如何与在患者中观察到的X染色体不平衡失活有关。

In humans, every chromosome is present in two copies, except in men, where there is one copy of each of the sex chromosomes, the X and the Y. Women have two copies of the X chromosome. One of the two X chromosomes in women is inactivated (silenced), so that male and female cells have the same amounts of the gene products coming from the X chromosomes. This process of silencing is called X-inactivation. Either of the two X chromosomes can be inactivated randomly. This means that, in every tissue, half of the cells would have one X chromosome inactive (A) and half would have the other one inactive (B). How is the random choice of one of the two X chromosome initially made? What are the mechanisms that inform one chromosome that other has been chosen? How is a different choice stably maintained in the progeny of each cell, throughout the life of an individual? These are all still unanswered or only partially-answered questions. In addition, in reality, the two copies of the X chromosome are never completely identical and the small differences in their DNA sequences can result in one of the two copies being silenced more easily. As a consequence, for example, if it is easier to silence X chromosome A, then most of the cells in a human organ will have an active B X chromosome, rather than a mixture of cells with an active A and cells with an active B. This is very important when one of the two X chromosomes contains a completely or partially defective gene which can cause a disease. If the X chromosome that is preferentially silenced carries the disease version of the gene, the organ will be functional and the woman will be healthy. But if, instead, the X chromosome, which is preferentially silenced, is the one carrying the normal version of the gene, the organ will not be able to function properly and the woman will suffer from the disease. Identifying the features of the DNA sequences that drive this skew and understanding how X-inactivation works are therefore very important, as they will allow the early identification of the patients at risk of developing severe symptoms, among the women carriers of X-linked diseases. An example of such a disease, where skewed X-inactivation plays a role in the severity of the symptoms, is Duchenne muscular dystrophy, where some female patients are completely asymptomatic and others are severely ill.In our project, we use mouse embryonic stem cells to investigate three molecules, an RNA and two proteins, that we have shown to be essential for the initial choice of which X chromosome will be inactivated. By studying how these three molecules interact with each other and with the X chromosome DNA, we will identify the molecular mechanism that establishes the choice of the identity of the future active and inactive X chromosomes. In addition, we will explore the role of DNA sequences that can determine skewed X chromosome silencing and investigate how these sequences relate to the unbalanced X chromosome inactivation observed in patients.